Antigenicity of the surface A-layer protein of an atypical Aeromonas salmonicida strain was tested in goldfish following immunization with various A-protein (AP) antigenic preparations (soluble form, alum-precipitated, conjugated with polyacrolein nanoparticles, or bacterin). ELISA revealed innate specific anti-AP antibody activity in all fish on day 0, while no significant antibody enhancement was seen on days 21 or 42 following the primary immunization. A second stimulation with soluble AP resulted in a clear response in groups primed with bacterin and alum-precipitated AP but no response in those primed with soluble AP or AP-nanoparticles conjugate. The implication of these results for vaccine design is discussed.